1. Field of the Invention
The present invention relates to an optical semiconductor device such as an avalanche photodiode for optical communication, and more particularly, to an optical semiconductor device capable of improving an anti-moisture property.
2. Background Art
FIG. 18 is a perspective view showing a conventional optical semiconductor device. This optical semiconductor device is an avalanche photodiode for optical communication (e.g., see Japanese Patent Laid-Open No. 10-209486).
An n-type InGaAs light absorbing layer 12, an n-type InP layer 13 which is a window layer and a multiplication layer are multilayered one atop another on an n-type InP substrate 11. By selectively diffusing impurities and implanting ions, a p-type InP region 14 is formed on a part of the top surface of the n-type InP layer 13. The top surfaces of the n-type InP layer 13 and the p-type InP region 14 are covered with a surface protection film 15. A cathode electrode 16 is connected to the underside of the n-type InP substrate 11. A ring-shaped anode electrode 17 is connected to the top surface of the p-type InP region 14. The region surrounded by this anode electrode 17 is a light receiving section 18.
Next, the operation of the above described optical semiconductor device will be explained. A voltage lower than that of the cathode electrode 16 is applied to the anode electrode 17. That is, a reverse bias is applied between the anode electrode 17 and the cathode electrode 16. This reverse bias (operating voltage) is adjusted to be approximately 90% of a breakdown voltage. Since the breakdown voltage is quite high on the order of 20 to 80 V, the reverse bias is as high as a maximum of approximately 70 V.
A light signal enters the light receiving section 18 from above in the figure. Since a p-type InP layer 63 has a large band gap, it allows light of a wavelength (1.3 μm and 1.55 μm) which is used in normal optical communication to pass without absorbing it. The light which has passed is absorbed by the n-type InGaAs light absorbing layer 12 having a small band gap, producing electrons and holes. These holes move through a depletion layer, enter the n-type InP layer 13 to which a high electric field is applied, provokes avalanche multiplication under such a high electric field and produces many new electrons and holes. As a result, a light signal is extracted from the optical semiconductor device as a multiplied current signal. This allows a signal having a current value more than ten times that when no multiplication occurs.